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1. Field of the Invention
The present invention relates to a method and a system for the application of radio frequency energy to products, such as agricultural commodities or valuable artifacts, in order to inhibit the presence of pests, bacteria, and other pathogenic or spoilage organisms. The present invention is also directed to the products treated with such radio frequency energy.
2. Description of the Background Art
Major human safety concerns exists today on the potential contamination food commodities with pathogenic bacteria such as Escherichia coli O157:H7, Salmonella sp., Listeria, and especially Campylobacter (see, for example, J. L. Welbourn: xe2x80x9cInside Microbiologyxe2x80x9d, in Food Testing and Analysis, pp. 20-22 Vol., 4 (3) June/July 1998). Each of these pathogenic bacteria has recently been identified as disease causing agents from the consumption of many common food commodities. Estimated food borne illness outbreaks and deaths annually in the United States affect 12 million and 4,000 individuals, respectively. Agricultural commodities such as grains, seeds, and spices may also be affected by fungal and/or bacterial contaminants. In addition, the keeping quality of agricultural commodities may also be affected by enzyme activity. Further still, microbial activity may generate a variety of toxins (i.e. Aflatoxin from Aspergillius flavus in grains).
Accordingly, it is desirable to inhibit the presence of disease-carrying organisms within food and agricultural commodities. Two manners of accomplishing this include slowing down the development of spoilage organisms (biostatic effects) or using thermal energy to cause a lethal effect on the organism (biocidal effect).
One manner of inhibiting the presence of such infective organisms, such as pathogens and insect/arachnid-type contaminants, is with thermal energy. The use of thermal energy to attack microorganisms within a host material is based on the fact that microorganisms will possess a greater sensitivity and vulnerability to thermal energy than the host materials. (i.e. agricultural commodities and other materials). This greater sensitivity is due to the greater complexity in the organism""s biological structure, as well as due to the existence of complex functional processes that are needed to sustain living organisms such as respiration, energy production, and cell division.
By way of illustration only, FIG. 1 depicts the relative sensitivities of host materials and infective organisms to thermal energy. Boundary 130 indicates the temperature at which irreversible changes occur in the host material. In FIG. 1, different classes of host material are allocated relatively different boundaries. Accordingly, fresh fruits are in region 131, plants in region 132, seeds in region 133, grains in region 134, and soils in region 135. FIG. 1 indicates that host material high boundary 130 is greatest (in a relative sense) in soils, and is lowest (in a relative sense) in fresh fruits. As used herein, xe2x80x9cirreversiblexe2x80x9d changes in the host material include (i) changes that affect the host material""s inherent metabolic and/or physiological attributes affecting the host material sensory and storage properties, or (ii) changes that affect the host material""s inherent chemical and molecular structure affecting the host material""s sensory and storage properties. For example, a host material that possesses some inherent metabolic activity is a green tomato following its harvest. A green tomato that is harvested and in conventional storage will continue to undergo metabolic changes associated with a color change (from green to red) and changes in chemistry that account for the taste of a ripe tomato. Accordingly, an irreversible change is a change that alters the host material""s inherent metabolic and/or physiologic attributes. For example, pickling vegetables or canning fruits alters the vegetable""s or fruit""s inherent metabolic activity and would, thus, be considered an irreversible change.
Alternatively, an example of a host material with a specific chemical or molecular structure that accounts for the host material""s sensory or storage properties is an artifact such as an antique book or an art object. In an antique book, the chemical or molecular structure of the ink on the page, or the molecular structure of the page itself accounts for the sensory properties associated with the book (i.e., color) as well as its potential value. Such molecular structure or chemical structure may be altered over a long period of time by the presence of spoilage organisms. Furthermore, the host material itself may be consumed by insects or mites. Accordingly, an irreversible change in such a host material is a change that alters the chemical or molecular structure of the host material so as to alter its sensory properties such as color, or its storage properties.
Further still, and in fresh fruits, inherent physiological properties include appearance, structure, and taste. Below boundary 130 (and above boundary 120) in FIG. 1, only xe2x80x9creversiblexe2x80x9d changes occur in the host material. Examples of reversible changes include such processes as small changes in temperature, where the temperature may cycle up and then down with no net change in the host material""s inherent metabolic and/or physiological attributes, or in the host material""s chemical or molecular structure as described above. Boundary 110 indicates the point above which irreversible changes occur in insects and arachnids. As used herein, xe2x80x9cirreversiblexe2x80x9d changes in infective organisms include changes that affect the organism""s ability to reproduce or the ability to survive. By way of illustration, below boundary 110 (and above boundary 100) reversible changes occur in insects and arachnids. Further still, boundary 120 indicates the region above which irreversible changes occur in microbes. Again, by way of illustration, below boundary 120 (and above boundary 110) reversible changes occur in microbes.
As stated above, thermal sensitivity in living matter is in direct proportion to biological complexity. Therefore, a high degree of biological complexity results in a high sensitivity to thermal energy. In FIG. 1, it is noted that insects and arachnids are the most sensitive, while soils are the least sensitive. The microbes depicted in FIG. 1 include fungi and yeasts, bacteria, viruses, and protozoa. Furthermore, and with respect to insects and arachnids in all life cycles, an induced thermal level of 40-60xc2x0 C. results in instant or delayed mortality or disruption of reproductive activity. When microorganisms are subjected to thermal energy only slightly above their maximum growth temperatures, an irreversible change, such as the reduction of viable cells or spores, generally follows. It is believed that this behavior is due to the denaturation of proteins, enzymes, or genes essential to reproduction. This is generally described in xe2x80x9cPhysical Principles of Food Preservation,xe2x80x9d part II, ed. Owen R. Fennema, Marcel Dekker Inc., 1975. Further still, although a valuable artifact such as an antique book or an art object may not have any xe2x80x9cbiological complexityxe2x80x9d as described above, the host material may be nevertheless highly sensitive to environmental factors, such as temperature, that may alter the host materials inherent chemical or molecular structure.
Accordingly, the application of thermal energy to a living-organism/host-material system, such as an infected food product or an infected artifact, can be utilized to target enzyme activity primarily and therefore the functional capabilities of living organisms. Enzyme inactivation is a critical goal in rendering a variety of products free of living contaminants such as insects, arachnids (i.e. mites), and microbes. The application of thermal energy to living organisms also imposes sub-lethal stress, which may lead to delayed mortality, and tissue damage due to the expansion of liquids.
Despite its usefulness in inhibiting the presence of pathogenic organisms, thermal energy is usually introduced on a limited basis to host materials such as fresh food or other artifact due to the irreversible changes introduced to the host material""s metabolic, physiological, chemical, molecular, sensory, or storage properties. One reason for this is that thermal energy is usually introduced through conduction, convection, and conventional microwave radiation. With the qualified exception of microwave radiation discussed below, an aspect of these conventional methods of introducing thermal energy is that one region of the host material, such as the surface, is initially exposed to more thermal energy than a neighboring region. This thermal energy, then, dissipates to the neighboring region through the process of conduction or convection. In all cases where this type of heat processing is used, it is necessary to apply a greater amount of thermal energy to one region in order to allow for heat transfer to be effective in distributing a sufficient amount of thermal energy to reach and control contaminating organisms over the entire product volume. As a result, heat applied to the host material through the selected region is often excessive and causes an irreversible change to that region, resulting in unacceptable damage.
Commercial applications for disinfection and/or disinfestation that attempt uniform thermal energy distribution are typically limited. An example of a technique used for a food product such as a mango is the hot water dip, which has varying results for the reasons discussed above.
Radio frequency (RF) radiation refers to electromagnetic radiation in the frequency range from approximately 3 kilohertz to 300 gigahertz. The ability of host material to absorb RF radiation generally varies as a function of frequency. FIG. 2 depicts an exemplary plot of absorption curve 200 of a host material versus frequency across a subset, for example, of the frequencies associated with RF radiation. A local maximum 210 at frequency f0 in the absorption curve identifies a frequency, conventionally understood as a xe2x80x9cresonantxe2x80x9d frequency, associated with a given host material. One skilled in the art should appreciate that the resonant frequency f0 is generally dependent upon the host material, including its geometry and dielectric properties. In resonance mode, RF energy is maximally transferred to the host material, providing a somewhat efficient transfer of energy. It can introduce thermal energy to a host material homogeneously and at controlled levels throughout the mass of the commodity.
Transferring thermal energy through RF radiation to a host material is different from processes that are based on conduction, convection, and conventional microwave-radiation. The dominant difference is due to the fact that RF processing can introduce thermal energy uniformly throughout the host material. In the conventional methods itemized above (with the exception of microwave radiation, discussed below), thermal energy is introduced to one region, for example, the surface, and is then transferred to the remaining regions through conduction or convection. Energy losses from the host material""s surface may be significant, requiring further thermal energy input in order to achieve the intended biocidal effect.
Unlike conduction and convection, however, the interaction between RF radiation and a host material and conventional microwave radiation and a host material is analogous. RF radiation, however, encompasses frequencies lower than the frequency of a conventional microwave oven, which is approximately 2,450 MHz. Because of this, RF radiation is able to generate thermal energy more homogeneously, deeper within a host material, and with less possibility of irreversible change to the host material.
Dipolar molecules within host material absorb both RF radiation and conventional microwaves. The differences between the effects that each have on the host material is due to their difference in frequency and wavelength. Conventional microwaves in a microwave oven have a frequency of approximately 2,450 MHz, and a wavelength of approximately 12.2 cm (approximately 4.8 inches). The separation between anti-nodes, therefore, is approximately one-half of a wavelength, or 6.1 cm (2.4 inches). Accordingly, and in a macroscopic object, the portions of the microwave radiation field that may not be imparting any energy to the macroscopic object are separated by approximately 6.1 cm or 2.4 inches. This accounts for the uneven heating ordinarily present in microwave ovens as well as for the typical practice of moving an item around inside a microwave cavity in order to achieve a semblance of uniform heating.
Alternatively, and considering RF radiation in the range of 100 MHz (approximately an order of magnitude lower in frequency than conventional microwaves), the wavelength of such RF radiation is approximately 300 cm (9.8 feet). Applying the same analysis as above, the separation between anti-nodes in such RF radiation is 150 cm (4.9 feet). Thus, the regions of uneven heating in such a RF radiation field are at most separated by 4.9 feet, which is well beyond the dimensions of a typical food product. Accordingly, RF radiation interacts more uniformly and deeply within a host material than does conventional microwave radiation. Such a general effect is recognized in U.S. Pat. No. 5,977,532, herein incorporated by reference.
The difference in frequency also accounts for the other major difference between RF radiation and microwave radiation: the fact that RF radiation is less likely to cause irreversible change in the host material than conventional microwave radiation. The frequency of microwave radiation is approximately one order of magnitude higher than RF radiation in the 100 MHz range. Accordingly, each photon that is absorbed and is not re-emitted (the primary means of energy absorption by a host material) imparts an order of magnitude more energy to the host material than does a photon of RF radiation in the 100 MHz range. This, of course, is desirable when one wishes to cook a food, since the whole goal of cooking is to introduce an irreversible change to the host material. However, conventional microwaves are undesirable when applied to fresh fruit, for example, and when one wishes no change in the qualities associated with freshness (such as appearance and taste).
This energy difference is also reflected in the formula that describes the absorption of RF radiation of the present invention by a host material. The power generated by RF radiation in a host material can be written:
P=55.61xc3x9710xe2x88x9214E2f∈xe2x80x3
Where: P is the power density generated in the host material (in W/cm3); E is the electric field strength (in V/cm); f is the RF frequency (in Hz); and ∈xe2x80x3 is the dielectric loss factor of the host material (dimensionless). The dielectric loss factor ∈xe2x80x3 is an intrinsic property of the host material. As stated above, and as is obvious from the above equation, a magnitude drop in frequency corresponds to a magnitude drop in transferred power at the same field strength.
Therefore, for most fresh agricultural commodities, microwave heating is not adequate, as it does not produce homogeneous heating. Microwave heating is also not homogeneous when large volumes of plant tissue are treated due to the rather limited penetration of 2,450 MHz photons. In addition, the high absorption of microwaves in water (a major component of fresh plant tissue) does not allow for low-level thermal treatments in a controllable manner.
The traditional or standard RF system used for RF heating is given in FIG. 3. The product sample 310 is placed inside Transversal Electromagnetic Cell 300 (TEM Cell 300). The RF wave travels across the cavity and interacts with sample 310. The remaining power exits on the opposite end and is measured as output power. The entire system operates in a single-pass transversal mode.
During this standard RF process, the RF input power Pi, the reflected power Pr, and the output power Po are measured. The flow of RF power exiting TEM Cell 300 (Po) is terminated in a heat sink cooled by forced air or a circulating coolant. Depending on how well the electromagnetic field interacts with the target, there are at least two possible outcomes.
In the first outcome, if there is no sample 310 in TEM cell 300, if the RF wave does not couple well, or if the RF wave hardly interacts with sample 310, the output power Po is roughly equal to the input power Pi and the reflected power Pr is roughly equal to zero. In this outcome, the absorbed RF power Pab may be written as:
Pab=[Pixe2x88x92Prxe2x88x92Po]=0
Thus, there is no energy transferred from the RF wave to the sample, and sample 310 is not actually heated.
In the second outcome, if an appreciable coupling exists between the RF wave and sample 310, an effective energy transfer from the RF wave to sample 310 will take place. In this outcome, the impedance of the RF system changes, the reflection power Pr increases and the output power Po is reduced. Accordingly, the absorbed RF power Pab may be written as:
Pab=[Pixe2x88x92Prxe2x88x92Po] greater than 0
In this situation, the sample temperature increases proportionally to the absorbed power Pab and this change may be expressed as:
xcex94T=Tfinxe2x88x92Tini
Where Tfin is the final temperature of the sample and Tini is the initial temperature of sample 310. The ratio of the absorbed power to the input power (Pab/Pi) is an important parameter that indicates the fraction of input RF power absorbed by sample 310. This absorbed/input power ratio Rab is given by:
Rab=Pab/Pi=1xe2x88x92(Pr+Po)/Pi
A high absorbed-power ratio Rab is desirable for best efficiency and lower cost. It also implies that a higher temperature differential (xcex94T) can be obtained for sample 310. These latter aspects allow processing with different thermal energy levels within the host material""s thermal window.
Experimental data indicates that the use of standard RF processing using the conventional RF system approach shown in FIG. 3 results in a maximum absorbed power ratio Rab of approximately 50-60%. Accordingly, the overall use efficiency and the temperature gradients available are both limited and low. Under the above conditions, commercial, large-scale uses of RF processing may be limited by both economic and practical considerations.
Accordingly, it is desirable to have a system that generates a high absorbed-power ratio Rab for use with RF processing.
Prior technology directed towards the incapacitation of infective organisms have tended to focus on the targeting of the organisms with electromagnetic radiation of power, intensity, and frequency sufficient to inhibit the microorganisms directly (non-thermal effects).
U.S. Pat. No. 4,524,079 to Hoffman et al. (the ""079 patent), herein incorporated by reference, teaches the use of an oscillating magnetic field in the frequency range between 5 kilohertz and 500 kilohertz in order to reduce microorganisms. One skilled in the art should appreciate that dynamic magnetic fields will induce electrical currents in tissues proportional to the change in the magnetic field and the conductivity of the tissue. The ""079 patent teaches that frequencies above 500 kilohertz are less effective in deactivating microorganisms by magnetic oscillation and will tend to heat the material, which is considered undesirable. The intensity of the applied field is disclosed in the ""079 patent as between 2 and 100 Tesla. Fields with intensities above 2 Tesla are generally accepted as having adverse effects on biological tissue. Furthermore, the magnetic field of the earth is at least 4 orders of magnitude smaller (approximately 10xe2x88x924 Tesla) than that disclosed in the ""079 patent.
U.S. Pat. No. 5,339,564 to Wilson et al. (the ""564 patent), herein incorporated by reference, teaches the use of frequency-hopping RF power (147 MHz and 240 MHz are examples of frequencies disclosed). The frequency is chosen to couple only to the natural polarization oscillations of animal mitochondria. The ""564 patent teaches that the frequencies do not harm plant cells because of their different structure. In addition, the ""564 patent states that dipole oscillations occur between 1 kilohertz and 1 megahertz, whereas the process of coherent excitation occurs at frequencies close to 100 MHz. The disclosed intensity at 147 MHz is 8 watts/m2.
U.S. Pat. No. 3,272,636 to Fehr et al. (the ""636 patent), herein incorporated by reference, teaches the use of a frequency range of 20 to 40 MHz, and intensity between 500 and 3000 volts per centimeter r.m.s. Again, the frequency chosen to be lethal to disease bearing microorganisms and destructive to the reproductive ability of organisms that causes food commodities to spoil without causing appreciable heat. The ""636 patent teaches that this frequency range does not cause internal heating of the food sufficiently to cook the food or change its flavor. The ""636 patent also teaches that lower frequencies could be used in instances where the food product is resistant to penetration by higher frequencies, or if the microorganism is more susceptible to lower frequencies. Furthermore, the ""636 patent teaches the use of higher frequencies up to the xe2x80x9cdielectric heating rangexe2x80x9d (1000 MHz) if additional heating or cooking of the food product is not important. As with the other references above, the ""636 patent teaches that the microorganisms are inhibited directly by the RF radiation at high power.
U.S. Pat. No. 2,485,660 to Robertson (the ""660 patent) discloses the use of plasma frequency emissions in the range of 1 MHz to 1000 MHz, with the preferred frequency being around 30 MHz or above. The frequency and power output are chosen to create an invisible corona discharge, which kills the living organism without appreciable heating of the surrounding media.
Accordingly, there is a need for a commercial process for causing irreversible changes to infective organisms while causing only reversible changes to the host materials such as: fresh fruits and vegetables; meat, poultry, and seafood; grains, seeds and spices; and valuable artifacts. This is due to the fact that fresh fruits, vegetables, and artifacts are normally affected by a heat-sensitive natural flora of spoilage organisms and, sometimes, as with fresh fruits and vegetables, experience an additional contamination with pathogenic organisms (bacteria) due to handling and packaging. No conventional method based upon thermal energy is presently used. The application of RF radiation to grains, seeds, and spices has many objectives, as these host materials may be affected by fungal and/or bacterial contaminants. In all these cases, an RF method is able to provide a decontamination effect improving the general safety of these host materials. Furthermore, an RF method is able to preserve valuable artifacts such as antique books. Further still, thermal inactivation of enzymes promoting biochemical degradation of essential nutrients is a major application of the RF method leading to a better, non-chemical preservation technology for grains.
Accordingly, in a first embodiment of the present invention, a method for treating products includes introducing a radio frequency field determined by parallel-plate electrode geometry to a product containing a host material, where the radio frequency field is configured to resonantly introduce thermal energy to the host material at a frequency, where the thermal energy is sufficient to cause irreversible changes in infective organisms, and where the radio frequency field is configured at a power level such that the thermal energy causes only reversible changes in the host material.
In a second embodiment of the present invention, the system comprises a TEM Cell in which the terminating resistance is eliminated by: matching a product""s geometry with the electromagnetic field, forming a harmonic resonator with the commodity, and coupling the electromagnetic field with the product""s dielectric loss factor.
A third embodiment of the present invention comprises the host material treated by the method consistent with the first embodiment described above. Another embodiment of the present invention comprises: introducing a radio frequency field to a product comprising a host material where the radio frequency field is configured to be absorbed by the product at a rate less than approximately 500 watts for a time period between approximately 2 hours and 20 hours, and where the radio frequency field is configured to exhibit a frequency between approximately 800 kilohertz and 2 megahertz.
A further embodiment of the present invention comprises the host material treated by the above method.
Further still, another embodiment of the present invention comprises: a TEM Cell in which the terminating resistance is eliminated and the radio frequency radiation is configured to couple with the product such that: the output power and the reflected power are minimized, the radio frequency radiation is absorbed by the product at a rate less than approximately 500 watts for a time period between approximately 2 hours and 20 hours, and the radio frequency radiation is configured to exhibit a frequency between approximately 800 kilohertz and 2 megahertz.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.